In early February 2017, two plants of Peperomia tetragona (syn. P. puteolata), known as parallel peperomia, exhibiting signs of powdery mildew were sent from a hobby florist to H.D. Shin of Korea University for diagnosis. The peperomia plants were known to be pot-grown from April 2016 in the greenhouse and cared for in an indoor facility from October 2016 for ornamental purpose. Early signs of powdery mildew started as thin white colonies on both sides of young leaves and occasionally on tender stems from early January 2017. The infections were usually severe on young leaves (Fig. 1a, b), causing discolouration and premature defoliation and thus markedly detracting from the aesthetic value of the plant. A voucher specimen was deposited in the Korea University Herbarium (KUS-F29727).

Fig. 1
figure 1

Powdery mildew of Peperomia tetragona by Podosphaera xanthii. a Symptoms on leaves. b Close-up view of symptoms. c Conidiophore. d Conidia. e Conidia in germination

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To identify the powdery mildew associated with the infections, a fresh specimen was mounted in water in preparation for microscopic analysis. The morphological characteristics of the fungal structures were examined using bright-field and differential-interference contrast microscopy using an Olympus BX51 microscope (Olympus, Tokyo, Japan) for measurements, and with a Zeiss AX10 microscope equipped with an AxioCam MRc5 camera (Carl Zeiss, Göttingen, Germany) for photographs. At least 30 measurements were taken of each structure. Hyphae were sub-straight to flexuous, branched, and 4–7 μm in diameter. Hyphal appressoria were nipple-shaped or nearly absent. Conidiophores arising from the upper part of running hyphae were erect, 140–230 × 10–12 μm and produced 2 to 5 immature conidia in chains with a crenate outline (Fig. 1c). Foot-cells of conidiophores were straight, cylindric, and 50–80 μm long. Conidia were hyaline, ellipsoid-ovoid to barrel-shaped, 25–35 × 16–22 μm with a length/width ratio of 1.2–1.9, and contained distinct fibrosin bodies (Fig. 1d). Simple germ tubes were produced from the lateral position of conidia (Fig. 1e, f). No chasmothecia were observed until the leaves senesced and dropped. These structures are typical of the powdery mildew Euoidium anamorph of the genus Podosphaera. The specific measurements and morphological characteristics were consistent with those of P. xanthii (Braun and Cook 2012).

To confirm the initial identification, the internal transcribed spacer (ITS) regions of KUS-F29727 were amplified with primers ITS5/P3 as described in Takamatsu et al. (2009), and sequenced directly. The resulting 688 bp sequence was deposited in GenBank (Accession No. MF043939). A NCBI BLAST search of the Korean isolate showed 99% similarity with the sequences of P. xanthii isolates of various host genera (e.g., KJ698669, KP120971, KP120972, etc.). A phylogenetic tree was constructed with the ITS sequence generated in this study and 17 sequences of Podosphaera species retrieved from GenBank, using the Maximum Likelihood (ML) method in MEGA7 (Kumar et al. 2016). In a ML tree (Fig. 2), the Korean isolate on P. tetragona was clustered into a clade that contained P. xanthii from several hosts, as supported by Takamatsu et al. (2010). Thus, both the morphological characteristics and phylogenetic analysis support the identification of the newly collected powdery mildew fungus on P. tetragona as P. xanthii.

Fig. 2
figure 2

Phylogenetic relationship between Podosphaera xanthii isolates and some reference isolates retrieved from NCBI, inferred by the Maximum Likelihood method using the ITS regions. Bootstrap values(≥70%) based on 1000 replications are indicated above the branches. The scale bar represents 0.02 nucleotide substitutions per site. The isolate used in this work is indicated in bold

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Pathogenicity was confirmed through inoculation tests by gently pressing a diseased leaf onto young leaves of three asymptomatic, potted plants. Three non-inoculated plants were used as controls. Plants were maintained in a greenhouse at 24–30 °C. Inoculated leaves developed signs of powdery mildew after 9 days, whereas the control plants remained symptomless. The fungus present on the inoculated leaves was morphologically identical to that observed on the original infected leaves. The pathogenicity tests were performed twice with similar results.

The genus Peperomia accommodates as many as 1600 species spread in the tropical and subtropical regions of the world, though concentrated in Central America and northern South America (Mathieu and Posada 2006; Frenzke et al. 2015; Gutierrez et al. 2016). Numerous horticultural varieties have been commercialized and cultivated worldwide (http://www.peperomia.net/horticulture.asp). Nevertheless, occurrence of powdery mildews on Peperomia spp. seems to be rare. The only record was in Denmark as Oidium piperis on Peperomia verticillata (Amano 1986). According to USDA-ARS database (Farr and Rossman 2017), Oidium piperis has also been recorded on Piper betle from Asia (India, Myanmar, Sri Lanka, and Taiwan) and also on P. aduncum in Brazil. Braun and Cook (2012) supposed O. piperis to be a species of Podosphaera based on previous record of Sphaerotheca fuliginea s. lat. (now mostly belonging to Podosphaera xanthii) in Taiwan. Oidium piperis on P. aduncum in Brazil (Soares and Barreto 2005), however, does not belong to Podosphaera, since the conidia do not contain fibrosin bodies (Braun and Cook 2012). Nevertheless, there has been no molecular data for any specimens of powdery mildews on Peperomia and Piper plants. The current work provides the first ITS sequence data and proves the identity of a Podosphaera powdery mildew occurring on a piperaceous plant.

To our knowledge, this is the first report of powdery mildew caused by P. xanthii on P. tetragona globally. In commercial production of P. tetragona cuttings in plastic greenhouses, damages caused by an unidentified powdery mildew were found in May 2005 (data unpublished, samples not preserved, the fungus not identified, but several photos showing typical powdery mildew signs preserved by K.S. Han). Therefore, constant attention should be paid to powdery mildews during peperomia production.